The present invention relates to a method for controlling the transverse thickness profile of paper or other web-like product by controlling temperature level and/or the axial temperature profile of the mantle of a roll used in the manufacture of the paper or other web-like product, the roll forming a nip together with a counter-roll through which the paper or web-like product passes.
The invention also relates to a roll such as a calender roll, used in accordance with the method which comprises a cylindrical mantle rotatably journaled at its axial ends and within which a stationary inner shaft is situated.
It is known in the prior art to heat rolls of paper machines such, for example, as calender rolls, by supplying heat transfer liquid, usually water, into the interior of the roll mantle through a connection provided at one end of the shaft of the roll and by removing the heat transfer liquid through a connection provided in the opposite end of the roll shaft. The quantity Q of heat supplied to the roll in this manner can be designated by the following equation: EQU Q=m.multidot.c.multidot..DELTA.t=m.multidot.c.multidot.(t.sub.s -t.sub.u)
where m is the mass flow rate of the heat transfer liquid, c is the specific heat of the heat transfer liquid, t.sub.s is the temperature of the heat transfer liquid at the inlet to the roll and t.sub.u is the temperature of heat transfer liquid at the outlet from the roll. When water is used as the heat transfer liquid, the maximum value of t.sub.s is about 120.degree. C. If t.sub.s is higher than about 120.degree. C., oil is generally used as the heat transfer liquid. However, the specific heat c of oil is less than that of water.
Prior art arrangements of the type described above have several drawbacks. For example, one drawback is that a calender roll which is heated in the conventional manner described above must be dimensioned and constructed as a pressure vessel. From the viewpoint of the heating of a roll obtained by the conventional heating arrangement described above, it is a drawback that one axial end of the roll will be raised to higher temperature than the other end due to the temperature drop of the heat transfer liquid as it travels from the roll inlet to the outlet. Since it is necessary to increase the temperature differential (t.sub.s -t.sub.u) to increase the heating efficiency, an increase in efficiency will necessarily be accompanied by greater differences in temperature between the ends of the roll. It is a further drawback of conventional heating arrangements of the type described above that it is not possible using such arrangements to adjust the axial temperature profile of the roll.
Reference is also made to the publication OE-OS 29 02 955 of Escher Wyss AG of Switzerland which discloses apparatus for heating a variable-crown roll. In particular, a roll is disclosed in which nozzles are fitted in the space provided between an inner stationary shaft and an outer rotary mantle of the roll. Jets of heat transfer liquid are directed through the nozzles of the roll mantle for heating the same.